The Plessy v. Ferguson Supreme Court decision of 1896, which allowed state-sponsored segregation.
The south was mainly agricultural, cultivating cotton in some form was most people's entire livelihood, creating sort of a cultural divide from the north. plus, the menial and arduous labour in the south was incredibly appealing for slave owners, but this didn't happen in the north where factories and mills were everywhere and staffed by non enslaved people. cotton united the two by the south growing it and the north turning it into textiles and other finished products. together they had quite a good system since each part was able to specialize
